Method for establishing wideband communications through a time division switching system

ABSTRACT

A method of establishing an N time slot connection through first and second, single-buffered time slot interchangers where the time slots interconnecting the two time slot interchangers, referred to herein as network time slots, are chosen in accordance with particular selection criteria such that all N time slots received by the first time slot interchanger in a single frame, are also transmitted from the second time slot interchanger in a single frame.

TECHNICAL FIELD

This invention relates to telecommunications.

BACKGROUND AND PROBLEM

Most digital communication within the switched, telecommunicationsnetworks is limited to 64 kilobits per second due to the constraintsimposed by existing switching systems and transmission facilities. Inspecific applications requiring greater bandwidth, several narrowbandchannels are combined to form a wideband channel, sometimes referred toas an N×DS0 channel, where DS0 refers to a 64 kilobits per secondchannel and therefore an N×DS0 channel is an integer multiple of 64kilobits per second. A problem occurs when the data from groupedchannels is not switched through a switching system in the same orderthat the data is received. This occurs when a time slot interchanger ofthe switching system causes some, but not all, of the time slot data ofa given time division frame to be delayed and combined with the timeslot data of another time frame. One solution to this problem is toprovide double-buffered time slot interchangers where all the data fromone frame is written into a first buffer while at the same time data isread out from a second buffer that was stored therein during theprevious frame. The reading and writing of the first and second buffersalternates every frame. This solution works well but adds both the costof the additional memory and one frame of transmission delay for eachdouble-buffered time slot interchanger. In addition, the problem remainsfor the large number of existing switching systems with onlysingle-buffered time slot interchangers.

Solution

The above problem is solved and a technical advance is achieved inaccordance with the principles of the invention in a method ofestablishing an N time slot connection through first and second,single-buffered time slot interchangers where the time slotsinterconnecting the two time slot interchangers, referred to herein asnetwork time slots, are chosen in accordance with particular selectioncriteria such that all N time slots received by the first time slotinterchanger in a single frame, are also transmitted from the secondtime slot interchanger in a single frame.

A method in accordance with the invention establishes an N time slotconnection through first and second time slot interchangers where j1, j2. . . jN are the numbers associated with the N input time slots to thefirst time slot interchanger for the connection and k1, k2 . . . kN arethe numbers associated with the N output time slots of the second timeslot interchanger for the connection. For each integer i where ji>=ki, atime slot number xi is selected with xi>(ji+d) or xi<(ki-d) and xi is anidle output time slot of the first time slot interchanger and also anidle input time slot of the second time slot interchanger, and with dbeing a positive integer or zero. For each integer i where ji<ki, a timeslot number xi is selected with xi<(ji-d) or xi>(ki+d) and xi is an idleoutput time slot of the first time slot interchanger and also an idleinput time slot of the second time slot interchanger. Once the timeslots x1, x2 . . . xN are selected, the first time slot intercharger iscontrolled to connect input time slots j1, j2 . . . jN to the outputtime slots x1, x2 . . . xN of the first time slot interchanger. Thesecond time slot interchanger is controlled to connect the input timeslots x1, x2 . . . xN of the second time slot interchanger to the outputtime slots k1, k2 . . . kN.

When N time slots x1, x2 . . . xN are not available meeting the abovecriteria--i.e., a blocking condition, a further method may be used whereji>ki for all i. A time slot number xi<ji-d and xi>ki+d is selected foreach integer i with xi being an idle output time slot of the first timeslot interchanger and also being an idle input time slot of the secondtime slot interchanger. Alternatively, where ji<ki for all i, a timeslot number xi>ji+d and xi<ki-d is selected for each integer i with xibeing an idle output time slot of the first time slot interchanger andalso being an idle input time slot of the second time slot interchanger.

In the exemplary embodiment herein, the first and second time slotinterchangers are single-buffered. In some applications, the abovemethods will not result in sufficiently low blocking probabilities. Onesolution to this is make the input time slots j1, j2 . . . jN and/or theoutput time slots k1, k2 . . . kN selectable by interposing a third timeslot interchanger in front of the first time slot interchanger and/orinterposing a fourth time slot interchanger after the second time slotinterchanger. The third and fourth time slot interchangers aredouble-buffered. To reduce blocking, the selections are made such thatfor each integer i, the time slot number ji is as close as possible tothe time slot number ki.

An alternative expression of the basic method of the invention relies onthe definition for each integer i from 1 to N, of si as the smallernumber of the two numbers ji and ki, and li as the larger number of thetwo numbers ji and ki. The N time slots x1, x2 . . . xN are selectedsuch that for each xi, a data word received by the first time slotinterchanger in time slot ji of a first frame of time slots is connectedthrough to a second time slot interchanger and transmitted therefrom intime slot ki of a second frame of time slots immediately following thefirst frame. The data word is transmitted from the first time slotinterchanger in a time slot xi which is between time slot (li+d) of thefirst frame and time slot (si-d) of the second frame, where di is apositive integer or zero.

DRAWING DESCRIPTION

FIGS. 1-3 are diagrams of a prior art switching system;

FIG. 4 is a block diagram of a switching system utilizing an exemplarycall processing method of the present invention;

FIG. 5 and 6 illustrate time slot selection intervals useful inperforming the path hunt function in the switching system of FIG. 4;

FIG. 7 illustrates the results of the exemplary path hunt method of thepresent invention for a 4×DS0 call;

FIG. 8 is a flow chart of the exemplary path hunt program of the presentinvention;

FIG. 9 is an alternative embodiment of the system of FIG. 4; andillustrating an example call between two primary rate interfaces;

FIG. 10 is a flow chart of an exemplary path hunt program as applicableto the example call of FIG. 9;

FIG. 11 is the same FIG. 9 alternative embodiment illustrating anexample call between a primary rate interface and a DS1 trunk; and

FIG. 12 is a flow chart of an exemplary path hunt program as applicableto the example call of FIG. 11.

DETAILED DESCRIPTION

The following description is arranged in two parts: (1) the AT&T 5ESS®switch is described as it exists in the prior art; and (2) callprocessing in an exemplary method of the invention is described in termsof departures over the prior art system.

Prior Art System 1000

FIGS. 1-3 are used to describe the prior art switching system 1000. TheAT&T Technical Journal, July-August 1985, Vol. 64, No. 6, Part 2, U.S.Pat. No. 4,322,843 issued to H. J. Beuscher et al. on Mar. 30, 1982,U.S. Pat. No. 4,683,584 issued to S. Chang et al. on Jul. 27, 1987 andU.S. Pat. No. 4,621,357 issued to S. Naiman et al. on Nov. 4, 1986describe aspects of the system in detail.

Switching system 1000 (FIG. 1) has three major components: anadministrative module (AM) 4000 that provides systemwide administration,maintenance, and resource allocation; a communications module (CM) 2000that provides a hub for distributing and switching voice or digitaldata, control information, and synchronization signals; and a number ofswitching modules (SMs) 3000-1, 3000-N that perform local switching andcontrol functions and that provide interfaces to subscriber lines andinterexchange circuits.

AM 4000 provides the system-level interfaces required to operate,administer, and maintain system 1000. It performs functions that canmost economically be done globally, such as common resource allocationand maintenance control. For reliability, AM 4000 includes fullyduplicated processors and the two processors work in an active/standbyconfiguration. In normal operation the active processor has control and,at the same time, keeps the data in the standby processor up to date.Thus when a fault occurs in the active processor, the standby processoris switched into service with no loss of data.

AM 4000 performs many call-processing support functions, includingsystemwide craft maintenance access, diagnostic and exercise control andscheduling, software recovery and initialization, and certainfault-recovery and error-detection functions best done on a centralizedbasis. Within AM 4000, there is error-checking circuitry for detectingand isolating faults. AM 4000 also performs administrative functions andprovides software access to external data links and to disk storage (notshown).

The basic function of CM 2000 (FIG. 2) is to provide consistentcommunications between the SMs, and between AM 4000 and the SMs. Amessage switch (MSGS) 2020 transfers call-processing and administrativemessages between the SMs and AM 4000, and between any two SMs. MSGS 2020performs a packet-switching function within system 1000 utilizing thewell-known X.25 level-2 protocol to transfer control messages through CM2000 and its terminating network control and timing (NCT) links 100-1,100-N. This protocol includes error detection, positive messageacknowledgment, and message retransmission in the event of atransmission error. A network clock 2030 provides the clock signals thatsynchronize the time-division network. Clock 2030 is synchronizedthrough an external source or runs on an internal reference basis withperiodic updating.

System 1000 uses a time-space-time architecture. As illustrated in FIG.3, a time-slot interchange unit (TSIU) in each SM performs thetime-division switching; a time-multiplexed switch (TMS) 2010 in CM 2000(FIG. 2) performs the time-shared space-division switching. At eachinterface unit (FIG. 3) the outputs from lines and trunks are convertedinto 16-bit time slots. These bits are used for signaling, control, andparity, and for binary-coded voice or data. The time slots are switchedthrough the TSIU and time-multiplexed on NCT links to TMS 2010.

TMS 2010 (FIG. 2) is a single-stage switching network that provides thedigital paths for switched connections between the modules and forcontrol messages among modules. TMS 2010 interconnects the modules viathe NCT links. Each NCT link carries 256 channels (time slots) ofmultiplexed data in a 32.768-Mb/s serial bit stream. One of the timeslots carries control messages, and the remaining 255 time slots carrydigitized voice or data. Two NCT links are associated with eachswitching module, thus allowing 512 time slots to be routed to and fromTMS 2010. (However, only a single line 100-1 is shown in the drawing torepresent both NCT links between SM 3000-1 and CM 2000.) Setting up apath between a line or trunk on two SMs involves finding an idle timeslot on one of the NCT links to each SM. A path is then set up throughTMS 2010 between the two NCT links using the selected time slot. TheTSIU in each SM establishes a path between the selected NCT time slotand the peripheral time slot associated with the line or trunk. (Sincethe paths are bidirectional, one NCT time slot is needed for eachdirection of transmission. In the present embodiment however, the timeslots for the two directions are selected to have the same number.)

One of the signaling bits of the 16-bit time slots on the NCT links toTMS 2010, referred to as the E-bit, is used for continuity verificationbetween SMs after an inter-SM call has been set up through TMS 2010. Forexample, after a call between SM 3000-1 and SM 3000-N has been set upthrough TMS 2010 using a particular time slot, both SM 3000-1 and SM3000-N begin transmitting a logic one E-bit in the particular time slotas a continuity signal and both also begin scanning the E-bit of theparticular time slot received from the other SM. The call setupprocedure is not considered complete until both SM 3000-1 and SM 3000-Nhave detected the E-bit continuity signal from the other SM.

SMs such as SM 3000-1 (FIG. 3) provide call-processing intelligence, thefirst stage of switching network, and line and trunk terminals. SMsdiffer in the types and quantities of interface equipment they contain,depending upon the characteristics of the lines or trunks terminatingthereon. Certain equipment is however, common to all SMs. The commonequipment includes a link interface 3030, a TSIU 3010, and a modulecontrol unit 3020. Link interface 3030 provides a two-way interfacebetween each SM and TMS 2010 in CM 2000. Module control unit 3020controls call processing, call distribution, and maintenance functions.A variety of interface units 3041, 3042 are available in system 1000.Line units provide interfaces to analog lines. Trunk units provideinterfaces to analog trunks. Digital line trunk units provide interfacesto digital trunks and remote SMs, while digital carrier line unitsprovide the interface to digital carrier systems. Integrated servicesline units provide interfaces to digital ISDN lines. Each SM canaccommodate any mixture of these units, with up to 510 channels. Twotime slots are used for control.

TSIU 3010 includes a signal processor, which handles address andsignaling information and a control interface, which distributes controlsignals to and from the interface units. TSIU 3010 switches time slotsbetween the interface units in an SM and connects time slots from theinterface units to time slots on NCT links. TSIU 3010 switches 512 timeslots--256 from each of the NCT links between SM 3000-1 and CM 2000--and512 peripheral time slots from the interface units. TSIU 3010 canconnect any of its 512 peripheral time slots to any other peripheraltime slot, or to any time slot of either NCT link to CM 2000.

System 1000 is a time division switching system where the switchingfunction is distributed to the plurality of SMs 3000-1, 3000-N, eachconnected to a number of lines and/or trunks. Each SM providesconnections among the lines and trunks connected to that module. Callsinvolving lines or trunks connected to different SMs are completedthrough TMS 2010 that interconnects the SMs. Each SM includes a controlunit, e.g., module control unit 3020, that controls the switchingfunction of that SM. System 1000 also includes a central control, e.g.,AM 4000, that controls the switching function of TMS 2010. All callswithin system 1000 require the selection of what is referred to as anetwork time slot. For inter-module calls, the network time slot is usedfor transmission from one SM, through TMS 2010, to another SM. The samenetwork time slot is used for both directions of transmission. Forintra-module calls, the network time slot is used within the SM toconnect one line or trunk to another line or trunk. Two network timeslots are used for intra-module calls, one for each transmissiondirection. Although the call processing function is distributed insystem 1000 in that the real-time intensive tasks associated with calls,e.g., signal processing, are performed by the switching module controlunits, the functions of selecting the network time slot and setting upthe TMS 2010 path if the call is an inter-module call, are centralized,being performed by AM 4000.

Recall that there are 512 channels (time slots) TS0 through TS511between a given SM and TMS 2010 (FIG. 2) and that setting up a path foran inter-module call between SM 3000-1 and SM 3000-N involves finding achannel that is available on link 100-1, for example TS44, and that hasa corresponding available channel TS44 on link 100-N. AM 4000 stores anavailability bit map for each of the links 100-1 through 100-N for usein performing the network time slot selection function. For each timeslot marked not available on a given link, AM 4000 also storesinformation defining the connection through TMS 2010 to one of the otherlinks. Network time slots are again marked available and the connectioninformation deleted in AM 4000 after a call ends. (For reasons ofefficient processing, this operation may be deferred until apredetermined number of call disconnects, e.g., 15, occur or apredefined time elapses.) However, the path or connection through TMS2010 is not removed after the call ends. As described in theabove-referenced U.S. Pat. No. 4,621,357 of S. Naiman et al., TMS 2010removes connections only as necessary to establish a new connection. Theinformation defining established TMS 2010 connections is stored onlywithin TMS 2010 in system 1000, and the network time slot selectionfunction (also referred to herein as the available path selectionfunction) is performed without reference to such information.

Exemplary Method of the Invention

FIG. 4 is a block diagram of switching system 80 implementing anexemplary method of the present invention. The hardware elements ofsystem 80 are substantially the same as those of switching system 1000(FIGS. 1-3); however the drawing of FIG. 4 includes only thetime-multiplexed switch 30 portion of the communications module and thetime slot interchangers shown in FIG. 4 are unidirectional and aretherefore shown on either side of time-multiplexed switch 30 to aid inthe explanation of the present invention. One pair of unidirectionaltime slot interchangers corresponds to one of the bidirectionaltime-slot interchange units 3010 of FIG. 3.

Program 11 stored in administrative module 20 of system 80 is adapted toperform path hunts for N×DS0 calls. Note that the time slotinterchangers, e.g. 41, 42, in the system are single buffered. In orderto establish an N×DS0 call between the input time slots j1, j2 . . . jNof time slot interchanger 41 and the output time slots k1, k2 . . . kNof time slot interchanger 42, N idle network time slots x1, x2 . . . xNmust be determined. The selection intervals used in accordance with thepresent invention to select network time slot xi are shown in FIGS. 5and 6. FIG. 5 depicts Case 1 where ji>=ki. In Case 1, an idle networktime slot xi is selected either between 0 and ki-1 or between ji+1 and511. In Case 2 where ji<ki (FIG. 6), an idle network time slot xi isselected either between 0 and ji-1 or between ki+1 and 511. Moregenerally, the FIG. 5 intervals may be expressed as between 0 and ki-dand between ji+d and 511 and the FIG. 6 intervals may be expressed asbetween 0 and ji-d and between ki+d and 511, where d may be zero or apositive integer. In the present embodiment, d=1 because of the delaycharacteristics of the time slot interchangers. The exemplary method ofthe invention is usable in system 1000 (FIGS. 1-3) to select the networktime slot used for both transmission directions of an inter-module callas well as to select each of the two network time slots used for anintra-module call.

An alternative way of expressing the selection intervals is to denote sias the smaller number of the two numbers ji and ki, and li as the largernumber of the two numbers ji and ki. An idle network time slot xi isfound which is between time slot (li+d) of a first frame and time slot(si-d) of the second frame.

Call blocking may occur using the above method. When such blockingoccurs, a further method may be used if ji>ki for all i or if ji<ki forall i. If ji>ki for all i, network time slots xi may be selected, ifavailable, such that xi<ji-d and xi>ki+d. If ji<ki for all i, networktime slots xi may be selected, if available, such that xi>ji+d andxi<ki-d.

The path hunt results for a 4×DS0 call are shown in FIG. 7. Theselection intervals of Case 1 were applied for i=3, and x3=52 wasselected from the interval between 0 and 57. The selection intervals ofCase 2 were applied for i=1,2,4. Network time slot x1=4 was selectedfrom the interval between 0 and 11, network time slot x2=214 wasselected from the interval between 47 and 511, and network time slotx4=320 was selected from the interval between 241 and 511.

FIG. 8 provides the flow chart for the N×DS0 path hunt program inaccordance with the invention. Blocks 801 through 806 implement theprimary method with selection intervals as depicted in FIGS. 5 and 6.Blocks 807 through 814 implement the further method when the primarymethod is unsuccessful in obtaining N network time slots.

In some applications, the blocking probabilities obtained using theprogram of FIG. 8 may be unacceptably high. One solution to this is tomake either or both of the time slot sets ji and ki selectable. This maybe done through the use of additional time-slot interchangers, which arethemselves double-buffered such that they do not introduce any framemisalignment of time slots. As shown in FIG. 9, double-buffered timeslot interchangers are interposed between the single-buffered time slotinterchangers and primary rate ISDN interfaces. DS1 trunks are connecteddirectly to the single-buffered time-slot interchangers.

FIG. 10 is the path hunt program flow chart for an N time slotconnection between primary rate interfaces 61 and 62 of FIG. 9. In block851, the time slots j1, j2 . . . jN of time slot interchanger 41 and thetime slots k1, k2 . . . kN of time slot interchanger 42 are chosen suchthat for all i, ji is as close as possible to ki. Then in block 852 theprogram of FIG. 8 is performed.

FIG. 12 is the path hunt program flow chart for an N time slotconnection between primary rate interface 61 and output time slots k1,k2 . . . kN of time slot interchanger 43 for connection to DS1 trunk 63as shown in FIG. 11. In block 853, the time slots j1, j2 . . . jN oftime slot interchanger 41 are chosen such that for all i, ji is as closeas possible to ki. Then in block 854 the program of FIG. 8 is performed.

It is to be understood that the above-described embodiments are merelyillustrative of the principles of the invention and that many variationsmay be devised by those skilled in the art without departing from thespirit and scope of the invention. It is therefore intended that suchvariations be included within the scope of the claims.

We claim:
 1. A method of establishing an N time slot connection throughfirst and second time slot interchangers, with the output of said firsttime slot interchanger connected to the input of said second time slotinterchanger, where j1, j2 . . . jN are the numbers associated with theN input time slots to said first time slot interchanger for saidconnection and k1, k2 . . . kN are the numbers associated with the Noutput time slots of said second time slot interchanger for saidconnection, with i being a positive integer index i<=N, said methodcomprisingA) for each integer i where ji>=ki, select a time slot numberxi>(ji+d) or xi<(ki-d) with xi being an idle output time slot of saidfirst time slot interchanger and also being an idle input time slot ofsaid second time slot interchanger, and with d being a positive integeror zero, B) for each integer i where ji<ki, select a time slot numberxi<(ji-d) or xi>(ki+d) with xi being an idle output time slot of saidfirst time slot interchanger and also being an idle input time slot ofsaid second time slot interchanger, C) controlling said first time slotinterchanger to connect input time slots j1, j2 . . . jN to the outputtime slots x1, x2 . . . xN of said first time slot interchanger, and D)controlling said second time slot interchanger to connect the input timeslots x1, x2 . . . xN of said second time slot interchanger to outputtime slots k1, k2 . . . kN.
 2. A method in accordance with claim 1further comprisingE) where N time slots x1, x2 . . . xN are notavailable meeting the criteria specified in steps A) and B), and whereji>ki for all i, selecting, before step C), a time slot number xi suchthat xi<ji-d and xi>ki+d for each integer i with xi being an idle outputtime slot of said first time slot interchanger and also being an idleinput time slot of said second time slot interchanger, and F) where Ntime slots x1, x2 . . . xN are not available meeting the criteriaspecified in steps A) and B), and where ji<ki for all i, selecting,before step C), a time slot number xi such that xi>ji-d and xi<ki-d foreach integer i with xi being an idle output time slot of said first timeslot interchanger and also being an idle input time slot of said secondtime slot interchanger.
 3. A method in accordance with claim 1 wherein athird time slot interchanger is connected to the input of said firsttime slot interchanger, said method comprisingprior to step A),selecting for each integer i, the time slot number ji, with ji being anidle output time slot of said third time slot interchanger, andcontrolling said third time slot interchanger to connect N input timeslots to the output time slots j1, j2 . . . jN of said third time slotinterchanger.
 4. A method in accordance with claim 3 where for eachinteger i, the time slot number ji is selected to be as close aspossible to ki.
 5. A method in accordance with claim 3 where said firstand second time slot interchangers are single buffered and said thirdtime slot interchanger is double buffered.
 6. A method in accordancewith claim 1 wherein a third time slot interchanger is connected to theoutput of said second time slot interchanger, said methodcomprisingprior to step A), selecting for each integer i, the time slotnumber ki, with ki being an idle input time slot of said third time slotinterchanger, and controlling said third time slot interchanger toconnect the input time slots k1, k2 . . . kN of said third time slotinterchanger to N output time slots.
 7. A method in accordance withclaim 6 where for each integer i, the time slot number ki is selected tobe as close as possible to ji.
 8. A method in accordance with claim 6where said first and second time slot interchangers are single bufferedand said third time slot interchanger is double buffered.
 9. A method inaccordance with claim 1 wherein the output of a third time slotinterchanger is connected to the input of said first time slotinterchanger, the input of a fourth time slot interchanger is connectedto the output of said second time slot interchanger, said methodcomprisingprior to step A), selecting for each integer i, the time slotnumbers ji and ki, with ji being an idle output time slot of said thirdtime slot interchanger, ki being an idle input time slot of said fourthtime slot interchanger, controlling said third time slot interchanger toconnect N input time slots to the output time slots j1, j2 . . . jN ofsaid third time slot interchanger, and controlling said fourth time slotinterchanger to connect the input time slots k1, k2 . . . kN of saidfourth time slot interchanger to N output time slots.
 10. A method inaccordance with claim 9 where for each integer i, the time slots ki andji are selected to be as close as possible to each other.
 11. A methodin accordance with claim 9 where said first and second time slotinterchangers are single buffered and said third and fourth time slotinterchangers are double buffered.
 12. A method in accordance with claim1 with said output of said first time slot interchanger being connectedto said input of said second time slot interchanger via switch means.13. A method of establishing an N time slot connection through first andsecond time slot interchangers, with the output of said first time slotinterchanger connected to the input of said second time slotinterchanger, where j1, j2 . . . jN are the numbers associated with theN input time slots to said first time slot interchanger for saidconnection, k1, k2 . . . kN are the numbers associated with the N outputtime slots of said second time slot interchanger for said connection,with i being a positive integer index with i<=N, where for each integeri from 1 to N, si is the smaller number of the two numbers ji and ki andli is the larger number of the two numbers ji and ki, said methodcomprisingselecting N time slots x1, x2 . . . xN where for each xi, adata word received by the first time slot interchanger in time slot jiof a first frame of time slots is connected through to a second timeslot interchanger and transmitted therefrom in time slot ki of a secondframe of time slots immediately following said first frame, and wherethe data word is transmitted from said first time slot interchanger in atime slot xi which is between time slot (li+d) of said first frame andtime slot (si-d) of said second frame, where d is a positive integer orzero.
 14. A method in accordance with claim 13 with said output of saidfirst time slot interchanger being connected to said input of saidsecond time slot interchanger via switch means.